1. Field of the Invention
The present invention relates to a wafer etching method for locally etching relatively thick portions present on the surface of a wafer.
2. Description of the Related Art
In recent years, a wafer etching method has been devised which locally etches relatively thick portions on the surface of a silicon wafer etc. to thin the wafer and flatten the surface and thereby reduce the variation in shape, that is, the total thickness variation (TTV) or local thickness variation (LTV).
FIG. 7 is a schematic view of a wafer etching method of the related art.
In FIG. 7, reference numeral 100 represents a plasma generator. The plasma generator 100 causes plasma discharge of sulfur hexalluoride (SF6) to generate an activated species gas C containing fluorine (F) ions and radicals. This activated species gas C is sprayed onto the surface of a silicon wafer W from a nozzle 101.
The silicon wafer W is fixed on a stage 120. The stage 120 is made to move in the horizontal direction and the nozzle 101 made to scan the entire surface of the silicon wafer W, whereby portions Wa relatively thicker than the prescribed thickness on the surface of the silicon wafer W (hereinafter referred to as the xe2x80x9crelatively thick portionsxe2x80x9d) are led directly under the nozzle 101.
Due to this, the activated species gas G is sprayed on the protruding relatively thick portions Wa from the nozzle 101 and the relatively thick portions Wa are locally etched, whereby the surface of the silicon wafer W is flattened.
In this wafer etching method using SF6 gas, however, as shown by the hatching in FIG. 8, white turbidity B occurs at the surface of the silicon wafer W along the line of scanning A of the nozzle 101 at the time of etching. Not only is the surface of the silicon wafer W contaminated, but also the white turbidity inhibits uniform etching and can cause the TTV and LTV to become worse than desired.
As opposed to this, there is a wafer etching method which causes plasma discharge of carbon tetrafluoride (CF4). If this method is used, no white turbidity B is formed at the surface of the silicon wafer W, but the etching rate is much slower than the method using SF6 gas.
Therefore, a wafer etching method using SF6 which does not cause white turbidity has been desired.
As one example of such a technique, there is the technique shown in FIG. 9.
In this technique, the silicon wafer W is placed in a low atmospheric pressure environment of 1 Torr. A small discharge chamber 200 serving also as an electrode and filled with SF6 gas is brought close to a relatively thick portion Wa. In that state, plasma discharge of the SF6 gas is caused at a high frequency of 13.56 MHZ, whereby the relatively thick portion Wa is locally etched. At his time, since the plasma in the discharge chamber 200 is close to the relatively tick portion Wa, at the same time as the activated species gas G is etching the relatively thick portion Wa, the ions in the activated species gas G strike the relatively thick portion Wa. Therefore, it is believed the white turbidity is eliminated by the impact of the various types of ions and no white turbidity remains on the surface of the silicon wafer Wa.
In this wafer etching method of the related art shown in FIG. 9, however, since the ions in the activated species gas G strike the surface of the silicon wafer W, the crystal structure of the silicon wafer W is disturbed, impurities caused by the collisions of the various types of ions enter into the silicon wafer W, and a high quality mirror polish of the silicon wafer W might not be able to be achieved. Further, with this method, while it is not possible to discern the white turbidity visually under natural light the white turbidity can be seen under a condenser type lamp. It is not possible to completely prevent the occurrence of white turbidity by this method.
As opposed to this, the apparatus shown in FIG. 7 uses SF6 gas, has a plasma discharge position far away from the silicon wafer W, and sprays only activated species gas G to the silicon wafer W. Therefore, so long as the silicon wafer W is etched by this apparatus, no disturbance occurs in the crystal structure of the silicon wafer W. The present inventors used this apparatus and added hydrogen (H2) gas to the SF6 gas to locally etch the silicon wafer W, then inspected the surface of the silicon wafer W visually, whereupon they discerned no white turbidity. This is believed to be because the occurrence of white turbidity was suppressed by the presence of the hydrogen fluoride (HF) produced by the reaction between the fluorine (F) radicals and H2. Further, with this method, since the ions in the activated species gas G do not strike the silicon wafer W. the crystal structure of the surface portion of the silicon wafer W is not disturbed Further, it is believed that by heating the silicon wafer W to a predetermined temperature, it is possible to completely prevent the occurrence of white turbidity on the surface of the silicon wafer W.
Note that as shown in FIG. 10, there is a technique for etching the surface of a silicon wafer W by adding H2 gas to SF6 gas.
This technique arranges the silicon wafer W inside a high atmospheric pressure environment of 1500 Torr. A drum shaped electrode 300 longer than the diameter of the silicon wafer W is brought close to the silicon wafer W and the SF6 gas with the added H2 gas is interposed in the slight clearance between the electrode 300 and the silicon wafer W. In this state, plasma discharge of the gas is caused by a high frequency of 150 MHZ, whereby the surface of the silicon wafer W is etched. Further, in this technique as well, the plasma discharge position is made close to the silicon wafer W, so similar problems arise as with the technique shown in FIG. 9. Further, since the atmospheric pressure is high, the temperature of the gas rises. As a result, the temperature of the wafer rises and problems such as warping of the wafer occurs. Further, it is not possible to suppress the occurrence of white turbidity.
An object of the present invention is to provide a wafer etching method which adds hydrogen gas, ammonia gas or mixed gas containing one of these gases to sulfur hexafluoride gas and thereby suppresses the occurrence of white turbidity at the surface of the wafer at the time of etching and enables a high quality mirror polish to the wafer.
To achieve this object, according to an aspect of he present invention, there is provided a wafer etching method comprising: a plasma generation step for converting sulfur hexafluoride gas to plasma at a discharge position in a discharge tube to generate; an activated species gas; and a spraying step for spraying the activated species gas onto a relatively thick portion of the wafer, in a state where a nozzle portion of the discharge tube leading the activated species gas generated at the discharge position to the wafer side is made to face the relatively thick portion of the wafer, so as to locally etch the relatively thick portion, wherein hydrogen gas, ammonia gas or mixed gas containing one of these gases is added to the activated species gas in a predetermined ratio.
Due to this configuration, in the plasma generation step, the sulfur hexafluoride gas is converted to plasma at the discharge position in the discharge tube to generate an activated species gas. Further, in the spraying step, the activated species gas generated at the discharge position is led by the nozzle portion to the wafer side and the activated species gas from the nozzle portion facing a relatively thick portion of the wafer is sprayed to the relatively thick portion whereby the relatively thick portion is locally etched. At this time, since the wafer is etched by the activated species gas comprised primarily of the sulfur hexafluoride gas, the etching rate is extremely high. Further, since the activated species gas is sprayed for etching from a nozzle portion away from the discharge position where the plasma is generated, the crystal structure of the wafer surface is not disturbed. Further, since hydrogen gas, ammonia gas or mixed gas containing one of these gases is added in a predetermined ratio to the activated species gas, the occurrence of white turbidity on the wafer surface can be suppressed.
Any method may be used to add the hydrogen gas, ammonia gas or mixed gas containing one of these gases to the activated species gas. As one example, according to an aspect of the invention, a mixed gas comprising the sulfur hexafluoride gas in which hydrogen gas, ammonia gas or mixed gas containing one of these gases is mixed at a predetermined ratio is fed to the discharge position in the discharge tube to generate an activated species mixed gas containing the activated species gas and the activated species mixed gas is sprayed from the nozzle portion.
Further, according to an aspect of the invention, the hydrogen gas, ammonia gas or mixed gas containing one of these gases is fed at a predetermined ratio from a gas feed pipe connected near the nozzle portion to the inside of the nozzle portion to generate an activated species mixed gas containing the activated species gas and the activated species mixed gas is sprayed from the nozzle portion.
Further, to maintain the etching rate at the desired level, it is preferable to etch by just the activated species gas comprised of the sulfur hexafluoride as much as possible. Therefore, according to an aspect of the invention, the activated species mixed gas is sprayed from the nozzle portion so as not to strike the wafer and to make the area around the wafer an atmosphere of the activated species mixed gas, then just sulfur hexafluoride gas is converted to plasma to generate an activated species gas for etching the wafer.
Any ratio of the hydrogen gas, ammonia gas or mixed gas containing one of these gases may be used, but as a good example, according to an aspect of the invention, the hydrogen gas, ammonia gas or mixed gas containing one of these gases is added in an amount of between 0.1 to 30 percent with respect to the mixed gas or the activated species mixed gas.
Further, a high precision of local etching would be possible if it were possible to suppress the dispersion of the activated species gas sprayed from the nozzle portion. Therefore, according to an aspect of the invention, hydrogen gas, ammonia gas or mixed gas containing one of these gases is filled around the activated species gas sprayed from the nozzle portion.
Due to this configuration, it is possible to suppress the dispersion of the activated species gas by the hydrogen gas, ammonia gas or mixed gas containing one of these gases and possible to make the diameter etched by the activated species gas a desired size.
Further, according to an aspect of the invention, there further comprises a heating step for heating the surface of the wafer to a predetermined temperature.
Due to this configuration, it is possible to substantially completely suppress the occurrence of white turbidity to an extent where white turbidity cannot be visually discerned even under a condenser type lamp.
Further, as a good example of the wafer heating temperature, according to an aspect of the invention, the heating temperature of the wafer in the heating step is set to a temperature between 60xc2x0 C. to 170xc2x0 C.